On SBI, there are three ways to do this, getting increasingly cheaper depending on how many hoops you're willing to jump through.
- 0.25 yen / dollar: Automatically convert to yen when selling. 1000 USD costs 250 JPY. Actually I'm not sure about this one, but the FAQ just says that their standard spread applies, which I assume is the same as the below method. (source).
- 0.25 yen / dollar: Use SBI Security's built-in currency converter to place a currency conversion order after selling. 1000 USD costs 250 JPY. Out of ignorance, this is what I used to convert to USD in the first place. (source).
- 0.04 yen / dollar: Transfer to SBI Net Bank and convert there. 1000 USD costs 40 JPY. (source)
SBI also have three other currency exchange platforms, but they only support yen transfers in and out for margin trading, so are a waste of time for converting proceeds of US stocks. I got a bit lost and wasted some time opening an FX account to discover this. I found this (Japanese) article which helped me gather the above information:
【外貨の米ドルから円への両替の方法】 SBI証券から住信SBIネット銀行へ外貨出金